4of 4Migrants in the caravan rest in tents at a fair venue in Tapachula. Trump said Monday he has alerted the military.Photo: PEDRO PARDO;Pedro Pardo / AFP / Getty Images

WASHINGTON — President Trump ratcheted up his rhetoric Monday about a caravan of thousands of Central Americans making its way toward the U.S., even as uncertainty grew over what will happen to the migrants if they reach the border.

Trump has seized on the caravan as a key talking point heading into the midterm elections. The president has been pointing to the growing group of migrants as justification for his aggressive immigration proposals.

“Sadly, it looks like Mexico’s Police and Military are unable to stop the Caravan heading to the Southern Border of the United States. Criminals and unknown Middle Easterners are mixed in. I have alerted Border Patrol and Military that this is a National Emergy. Must change laws!” Trump tweeted Monday.

Sadly, it looks like Mexico’s Police and Military are unable to stop the Caravan heading to the Southern Border of the United States. Criminals and unknown Middle Easterners are mixed in. I have alerted Border Patrol and Military that this is a National Emergy. Must change laws!

A source familiar with the government’s information on the caravan said there was no evidence “Middle Easterners” were mixing into it. It’s unclear whether Mexico will allow the group to continue the remaining 1,000-plus miles to the U.S. border without interfering.

A large factor attracting migrants is potential safety in numbers. It is virtually impossible to travel through Mexico to the U.S. border without paying smugglers, as the organized crime cartels control territory that migrants need to cross. On the way, the migrants are often abused, extorted and assaulted. Caravans offer an opportunity to travel together and avoid some of those dangers.

Caravans have also been seen as an opportunity for migrants and their supporters to call attention to their plight. There is extreme violence and poverty in what’s known as the Northern Triangle — Honduras, Guatemala and El Salvador — which remains the largest driver of migrants to the U.S. border.

Q: What happens when they reach the U.S.?

It is impossible to predict how many of the migrants will eventually reach the border. In the past, hundreds of migrants have fallen away from caravans as they traveled, either turned back by Mexico, settling there, or setting off on their own.

The migrants usually intend to turn themselves in to authorities in the U.S. Once they’re at a designated port of entry or if they are apprehended trying to cross illegally, they can tell officials that they have a fear of persecution in their home country. That triggers an asylum interview.

Q: Where will they arrive?

A caravan last spring ended up in Tijuana, so most of the migrants tried to cross at the San Ysidro port of entry south of San Diego.

That border crossing is the busiest in the hemisphere and regularly has long wait times for migrants seeking asylum. In April, some migrants had to wait there for weeks.

Customs and Border Protection Commissioner Kevin McAleenan testified at a congressional hearing in July that the wait at that crossing at the time was 1,000 people long. The port can process 50 to 100 people a day, he said.

Q: So will they be let into the U.S.?

It depends. By law, asylum seekers must be given a chance to make their case once they set foot on U.S. soil. But the U.S has been stopping people before they get to the border, which officials call the “limit line,” until space is available. There is a shelter network on the Mexico side of the border.

An inspector general report analyzing the administration’s handling of the family separation crisis this summer faulted that “metering” for causing more people to cross into the U.S. illegally after they were turned away from the port of entry.

Long waits could also worsen an already dangerous situation, said Eric Olson, an expert on Mexico and Latin America at the Wilson Center, a nonpartisan institute in Washington. Cartels target desperate migrants at the border, he said, and homicide rates have skyrocketed there this year.

“You introduce more chaos, more uncertainty to those areas and you’re going to exacerbate a very difficult humanitarian situation,” Olson said “There are very active criminal organizations operating in those areas that specialize in moving migrants, and they are going to certainly take advantage of the situation.”

Q: What happens once they’re here?

Once on U.S. soil, either legally at a crossing or caught illegally, an immigrant claiming fear of persecution back home is interviewed to assess whether that fear is “credible.” Those who pass that screening are given a court date, often years in the future, at which they can make their case for asylum. Those found not to have a credible fear are put into expedited deportation proceedings.

Most immigrants waiting for asylum proceedings are let out of detention, often with tracking devices. The Trump administration, however, has sought to be able to detain immigrants for longer and to move their cases through faster.

Q: Is the U.S. preparing for their arrival?

The Department of Homeland Security did not respond to a request for comment about preparations for the caravan. In a statement, Secretary Kirstjen Nielsen said the U.S. would work with other countries to prosecute criminal organizations that “prey” on migrants.

Ur Jaddou, a former counsel at U.S. Citizenship and Immigration Services in the Obama administration who now works for pro-immigration advocacy group America’s Voice, said the administration should be getting ready.

Instead, Jaddou said, Trump is “stoking fear, tweeting about it. ... He’s using this to scare people rather than seeing it as, ‘We have a problem, we should address it appropriately,’ as a good government should do.”

Q: What are the migrants’ chances of being granted asylum?

Historically, roughly 80 percent of migrants pass their initial credible-fear interview, but fewer end up winning their legal case for asylum. To stay in the U.S., asylum seekers must prove to an immigration court that they are being persecuted for something they cannot change, and that their government is unable or unwilling to protect them.

But the right to claim asylum is not something the Trump administration can change by itself. That is enshrined in both domestic and international law, and would require legislation in Congress to change.

Q: What about families and children?

Laws and court settlements require that immigrant children be held in custody for only short periods — either three days by themselves or 20 days as part of a family. Children who arrive alone must be handed over to the Department of Health and Human Services, which runs a shelter network that houses the children until they can be placed with an adult in the U.S.

The Trump administration separated more than 2,500 families to prosecute the parents this spring, then backtracked amid an international outcry. Since June, families caught crossing the border illegally have been held and processed together and most have been released into the U.S. with monitoring if they pass their asylum interview.

Tal Kopan is the Washington Correspondent for The San Francisco Chronicle. Previously, she was a political reporter for CNN Politics, where she covered immigration, cybersecurity and other hot-button issues in Washington, including the 2016 presidential election.

Prior to joining the network, Kopan was a reporter for POLITICO in Washington, D.C., where she reported for their breaking news team and policy verticals. While covering policy, she was a reporter for POLITICO Pro Cybersecurity, where she covered cybersecurity policymaking on Capitol Hill and followed cyber-related issues in the Justice Department such as cybercrime.

Kopan also previously worked as a Web producer at Fox Chicago News and as a freelance Web producer at ABC 7 Chicago, where she spent time covering stories such as the trials of former Illinois Gov. Rod Blagojevich and the election of Chicago Mayor Rahm Emanuel.

Kopan was selected as a 2014-2015 National Press Foundation Paul Miller Fellow and a member of the 2015 class of Journalist Law School at Loyola Law School, Los Angeles. She was also the recipient of the National Academy of Television Arts and Sciences, Midwest Chapter's Ephraim Family Scholarship.

Kopan graduated with honors from the University of Chicago with a bachelor's in law, letters and society.